1. Field of Invention
The present invention relates generally to the field of semiconductor optical emission devices, more particularly to a method for fabricating highly efficient and cost effective InAlGaN devices.
2. Description of Related Art
Sapphire has proven to be the preferred substrate for growing high efficiency InAlGaN light emitting devices because of its stability in the high temperature ammonia atmosphere of the epitaxial growth process. However, sapphire is an electrical insulator with poor thermal conductivity resulting in unusual and inefficient device designs. A typical LED structure grown on sapphire has two top side electrical contacts and a semitransparent metal layer to spread current over the p-contact. This contrasts with the standard vertical structure for current flow in LEDs grown on conducting substrates such as GaAs or GaP in which an electrical contact is on the top side of the semiconductor device and one is on the bottom. The two top side contacts on the sapphire based LED reduce the usable light emitting area of the device.
Furthermore, the low conductivity of the p-type InAlGaN layer results in the need for a semitransparent metal layer to spread current over the p-type semiconducting layer. The index of refraction of the sapphire (n˜1.7) is also lower than that of the InAlGaN layers (n˜2.2-2.6) grown upon it. Consequently, this mismatch in index of refraction (with the Sapphire being lower) results in waveguiding of the light between the absorbing semitransparent p-side current-spreading metallization and the sapphire. This results in absorption of 10-70% of the light generated in commercial InAlGaN device by the semitransparent metal layer.
Wafer bonding can be divided into two basic categories: direct wafer bonding, and metallic wafer bonding. In direct wafer bonding, the two wafers are fused together via mass transport at the bonding interface. Direct wafer bonding can be performed between any combination of semiconductor, oxide, and dielectric materials. It is usually done at high temperature (>400° C.) and under uniaxial pressure. One suitable direct wafer bonding technique is described by Kish, et al, in U.S. Pat. No. 5,502,316. In metallic wafer bonding, a metallic layer is deposited between the two bonding substrates to cause them to adhere. This metallic layer may serve as an ohmic contact to either the active device, the substrate or both. One example of metallic bonding is flip-chip bonding, a technique used in the micro- and optoelectronics industry to attach a device upside down onto a substrate. Since flip-chip bonding is used to improve the heat sinking of a device, removal of the substrate depends upon the device structure and conventionally the only requirements of the metallic bonding layer are that it be electrically conductive and mechanically robust.
A vertical cavity optoelectronic structure is defined to consist of an active region that is formed by light emitting layers interposing confining layers that may be doped, un-doped, or contain a p-n junction. The structure also contains at least one reflective minor that forms a Fabry-Perot cavity in the direction normal to the light emitting layers. Fabricating a vertical cavity optoelectronic structure in the GaN/InxAlyGazN/AlxGa1−xN (where x+y+z=1.0) material systems poses challenges that set it apart from other Ill-V material systems. It is difficult to grow InxAlyGazN structures with high optical quality. Current spreading is a major concern for InxAlyGazN devices. Lateral current spreading in the p-type material is ˜30 times less than that in the n-type material. Furthermore, the low thermal conductivity of the substrates adds complexity to the device design, since the devices should be mounted p-side down for optimal heat sinking.
One vertical cavity optoelectronic structure, e.g. a vertical cavity surface emitting laser (VCSEL), requires high quality mirrors, e.g. 99.5% reflectivity. One method to achieve high quality mirrors is through semiconductor growth techniques. To reach the high reflectivity required of distributed Bragg reflectors (DBRs) suitable for VCSELs (>99%), there are serious material issues for the growth of semiconductor InxAlyGazN DBRs, including cracking and dopant incorporation. These mirrors require many periods/layers of alternating indium aluminum gallium nitride compositions (InxAlyGazN/Inx′Aly′Gaz′N). Dielectric DBRs (D-DBR), in contrast to semiconductor DBRs, are relatively straightforward to make with reflectivities in excess of 99% in the spectral range spanned by the InxAlyGazN system. These mirrors are typically deposited by evaporation or sputter techniques, but MBE (molecular beam epitaxial) and MOCVD (metal-organic chemical vapor deposition) can also be employed. However, only one side of the active region can be accessed for D-DBR deposition unless the host substrate is removed. Producing an InxAlyGazN vertical cavity optoelectronic structure would be significantly easier if it was possible to bond and/or deposit D-DBRs on both sides of a InxAlyGazN active region.
In “Low threshold, wafer fused long wavelength vertical cavity lasers”, Applied Physics Letters, Vol. 64, No. 12, 1994, pp. 1463-1465, Dudley, et al. taught direct wafer bonding of AlAs/GaAs semiconductor DBRs to one side of a vertical cavity structure while in “Room-Temperature Continuous-Wave Operation of 1.430 μm Vertical-Cavity Lasers”, IEEE Photonics Technology Letters, Vol. 7, No. 11, November 1995, Babic, et al. taught direct wafer bonded semiconductor DBRs to both sides of an InGaAsP VCSEL to use the large refractive index variations between AlAs/GaAs. As will be described, wafer bonding D-DBRs to InxAlyGazN is significantly more complicated than semiconductor to semiconductor wafer bonding, and was not known previously in the art.
In “Dielectrically-Bonded Long Wavelength Vertical Cavity Laser on GaAs Substrates Using Strain-Compensated Multiple Quantum Wells,” IEEE Photonics Technology Letters, Vol. 5, No. 12, December 1994, Chua et al. disclosed AlAs/GaAs semiconductor DBRs attached to an InGaAsP laser by means of a spin-on glass layer. Spin-on glass is not a suitable material for bonding in a VCSEL between the active layers and the DBR because it is difficult to control the precise thickness of spin on glass, and hence the critical layer control needed for a VCSEL cavity is lost. Furthermore, the properties of the spin-on glass may be inhomogeneous, causing scattering and other losses in the cavity.
Optical mirror growth of AlxGa1−xN/GaN pairs of semiconductor DBR mirrors with reflectivities adequate for VCSELs, e.g. >99%, is difficult. Theoretical calculations of reflectivity suggest that to achieve the required high reflectivity, a high index contrast is required that can only be provided by increasing the Al composition of the low-index AlxGa1−xN layer and/or by including more layer periods (material properties taken from Ambacher et al., MRS Internet Journal of Nitride Semiconductor Research, 2(22) 1997). Either of these approaches introduces serious challenges. If current will be conducted through the DBR layers, it is important that the DBRs be conductive. To be sufficiently conductive, the AlxGa1−xN layer must be adequately doped. Dopant incorporation is insufficient unless the Al composition is reduced to below 50% for Si (n-type) doping and to below 17% for Mg (p-type) doping. However, the number of layer periods needed to achieve sufficient reflectivity using lower Al composition layers requires a large total thickness of AlxGa1−xN material, increasing the risk of epitaxial layer cracking and reducing compositional control. Indeed, an Al0.30Ga0.70N/GaN stack ˜2.5 μm thick is far from sufficiently reflective for a VCSEL. Thus, a high reflectivity DBR based on this layer pair requires a total thickness significantly greater than 2.5 μm and would be difficult to grow reliably given the mismatch between AlN and GaN growth temperatures. Even though the cracking is not as great of an issue if the layers are undoped, compositional control and the AlN/GaN growth temperatures still pose great challenges to growing high reflectivity DBRs. Hence, even in applications where the DBRs do not have to conduct current, mirror stacks with reflectivities >99% in the InxAlyGazN material system have not been demonstrated. For this reason, dielectric-based DBR mirrors are preferred.
Semiconductor devices are manufactured many thousands to tens of thousands at a time on wafers. The wafers must be diced into individual die prior to packaging. If sapphire is used as the growth substrate one must thin and dice the sapphire substrate. The hardness and hexagonal crystal structure of sapphire make the dicing operation difficult and expensive.